Precision methods of producing accurately sized and shaped spherical objects or balls have been applied in the art of producing ball bearings for high performance mechanical operations. These manufacturing methods also have application in the electronics packaging art, especially in forming ball grid arrays, which are electronic packages having arrays of balls attached to the bottom of a substrate which contains computer chips or other components. In these applications, very small balls of approximately 0.022" must be applied to the bottom of a substrate in a dense array sometimes containing hundreds of these balls. The balls act as electrical contacts for connecting the ball grid array to a complementary electrical connector or another substrate and must all be exactly the same size and shape so as to achieve coplanarity along the entire mating surface of the ball grid array. In order to achieve a perfectly spherical ball in the manufacturing process, environmental factors such as gravity must be minimized or overcome.
Known methods of producing balls typically involve either screw machining or cold forming metal slugs into spheres. These methods typically repeatedly impinge the slugs onto a hard surface until the balls are spherically formed from the slugs. These known methods are unsatisfactory for precision applications since the ultimate size and shape of the balls are not precisely controllable. Additionally, the cost per ball produced is high, and it is desirable to produce precisely sized and shaped balls for a fraction of a cent each.
U.S. Pat. Nos. 2,980,628 and 3,023,171 to Smith disclose other methods of making spherical articles using a series of dropping tips in oil or gelling bath. While these methods result in substantially spherical objects, the end products are not perfect spheres and sometimes vary in size with respect to each other due to the inaccuracies of the dropping tips. The pressure and temperature of the fluid entering the dropping tips must be precisely controlled in order to achieve consistency in size of the end product balls.
U.S. Pat. No. 4,783,217 discloses a method and apparatus for producing spherical objects where the apparatus has a reservoir filled with a relatively dense molten material. The reservoir is heated and hydraulically pressurized to maintain the state of the dense material, and a second molten liquid of lesser density and higher melting point is injected into the reservoir. The pressure and surface tension in the reservoir acts to force the injected material into a spherical form and since the melting point of the dense molten material is lower than that of the injected molten material, the injected molten material will solidify as it rises through the dense molten material. This method is more precisely controllable due to the fact that an exact amount of molten material can be injected into the bath to repeatedly achieve a precise desired ball size. The problem with this device, however, is that the dense molten material must be maintained in a heated and pressurized state and the injected molten material must also be maintained at precise pressure and temperature conditions. This manufacturing method therefore requires heating of the entire apparatus and robust parts capable of withstanding the high temperatures necessary to maintain both metals in a molten state. It is therefore costly to manufacture the apparatus and also costly to operate it due to the heating and pressurizing requirements. What is needed is a simple apparatus which is relatively inexpensive to assembly and to operate, which is capable of mass producing precise sized and shaped balls.